Competition Rules

The competition is run by the ORGANIZING COMMITTEE.

User accounts

Anyone can create an account and post one or several challenges and/or break one or more challenges on the WhibOx Contest Edition 3 website.

The account owner may remain completely anonymous when creating an account. In addition to creating a login/password, an email address must be entered (in case the ORGANIZING COMMITTEE needs to contact the owner directly). However, the email address is not verified except for basic syntax. Users may e.g. enter their primary email address or, if they are willing to remain anonymous, may use a free service such as Mailinator to create an untraceable email address.

The same person may create several accounts. User accounts are password-protected but passwords CANNOT BE CHANGED and are NOT RECOVERABLE in case of loss. It is the users' responsibility to choose a strong enough password and to keep it in a safe place. Winners will be asked to log in and reveal their real name on the competition website if they want to be publicly awarded during the prize ceremony.

The website is public in the sense that anyone can freely browse it (including downloading challenges) but logging in is required to post or break challenges.

The competition website is referred to as the SYSTEM.

What is a challenge?

A challenge is a C source program that contains a function with prototype

void ECDSA_256_sign(unsigned char sig[64], const unsigned char hash[32]);

It implements an ECDSA signature algorithm on the NIST P-256 curve under some private key embedded into the code. The function ECDSA_256_sign takes two arguments:

  • hash : a pointer to the 32-byte hash of the message to be signed (the hash function is excluded from the implementation),
  • sig : the address for the output 64-byte signature.

The C language is the one accepted by GCC 10.2.0 with NO additional language dialect options. The source code must be completely portable and re-compilable towards any target architecture. In other words, it must be made only of generic C which in particular, excludes the use of inline assembly. Note that there is no restriction to a 32-bit architecture as in the first edition of the WhibOx contest. Native 64-bit operations are supported by default and contestants may use them as they see fit.

The source file is allowed to include the GMP library (version 6.2.1) through #include <gmp.h>. Besides that, no #include or extern is allowed in the source code, and more generally linking to external libraries (even the standard C libraries) is forbidden. Any attempt to maliciously attack the SYSTEM or the computer system of contestants will lead to DISQUALIFICATION. In case evidence of malicious code is found in a challenge, contestants are invited to report the issue to the ORGANIZING COMMITTEE directly ( If contestants are suspicious that some challenge program may contain malware, they may run GCC in docker or a virtual machine, exactly like the server does (using VirtualBox).

Documentation on how the server runs is found on GitHub. Contestants are invited to download and install their own instance of the server if they want to ensure that their challenge will be accepted when submitted.

IPR and copyright disclaimer: contestants may indicate (typically in a header in their source) which license applies to their challenge program, if any.

Requirements on the source and executable

  • The source code must be no bigger than MB.
  • The REFERENCE COMPILERS must use at most MB of RAM to complete compilation.
  • The compilation must not exceed seconds.
  • The following character strings are forbidden in the source code:
    • "#include", except #include <gmp.h>
    • "extern"
    • "_FILE__"
    • "__DATE__"
    • "__TIME"
    • "__STDC_"
    • "__asm__"
    • "(wsp)asm(nan)"
    where "(wsp)" indicates a white space and "(nan)" indicates any non-alphanumeric character. An occurence of either of these strings will lead to a rejection of the challenge program, even if they are embedded in (arbitrarily nested) quoted strings.
  • The executable must be MB in size or less,
  • The executable must use MB of RAM or less (this includes the stack and all global variables, at the exclusion of the code itself),
  • The executable must run in second or less per function call. This is not absolute time but CPU time, measured within a VirtualBox VM by the SYSTEM. Contestants may check that their challenges comply with this limitation by using the utility ulimit with the −t option.

The SYSTEM will reject a program that does not comply with these requirements.

Compliance with the requirements can be tested by downloading and running a local instance of the submission server found here.

Auxiliary inputs

A challenge must come with

  • the public key corresponding to the embedded private key,
  • a proof of knowledge of the private key.

The public key must be pasted into the appropriate field of the submission form as the 128-digit hexadecimal string encoding the 64-byte string \( \textsf{FE2OS}(x_Q) \parallel \textsf{FE2OS}(y_Q)\), where \( \textsf{FE2OS} \) is the "Field Element to Octet String" primitive defined in Section 3.1.3 of the BSI ECC guideline, \( (x_Q,y_Q) \) is the public key (\(x\)- and \(y\)-coordinates) and \( \parallel \) is the concatenation operator.

The proof of knowledge is a Schnorr signature of the empty message under the private key, that is a pair \( (r,s) \) such that \( \mathsf{SHA256}(s\cdot G+r\cdot Q)=r \), where \( G \) is the base point and \( Q \) is the public key. The input of \( \mathsf{SHA256} \) is the padded \(x\)-coordinate (32 bytes). The format of the proof of knowledge is similar to that of the public key. Namely, it is a 64-byte hexadecimal string obtained by concatenating \(r\) and \(s\) (each of them padded to 32 bytes).

A reference implementation with corresponding public key (formatted for submission) and a script to generate the proof of knowledge from the private key are available here.

Posting a challenge

A contestant who posts a new challenge must submit the corresponding public key to the SYSTEM for the purpose of verifying the consistency of the challenge program. This is done as follows.

Challenge-key verification procedure

  1. The SYSTEM verifies that the submitted public key has not been previously submitted. If it has been, the challenge is rejected.
  2. The SYSTEM verifies that the submitted public key is a valid public key by checking that the underlying elliptic point is on the curve P-256. If not, the challenge is rejected.
  3. The SYSTEM verifies the submitted proof of knowledge. If the verification fails, the challenge is rejected.
  4. The SYSTEM compiles the challenge program and checks that it complies with the above requirements. If not, the challenge is rejected.
  5. The SYSTEM generates a number of random input hashes (32-byte strings), uses the submitted implementation of ECDSA P-256 to generate the corresponding signatures, and uses the submitted public key to verify these signatures. The number of (input hash, signature) pairs is determined by the SYSTEM but is at least 100.
  6. In case of mismatch, the procedure halts, the challenge program is rejected and the mismatching (input hash, signature) pair is returned to the submitting contestant.
  7. Finally, the challenge program is validated and:
    1. given a name by the system,
    2. assigned a PERFORMANCE SCORE (see below)
    3. declared as STANDING,
    4. assigned a STRAWBERRY score (both initialized to 0).

Programmatic API access to input-output pairs

When accepted, the challenge is published on the website for anyone to download and play with. In addition, a small subset of random hash-signature pairs is kept by the SYSTEM. The number of pairs is determined by the SYSTEM but is at least 10. These pairs can be recovered as a JSON object with the following HTTP request:

GET https://{HOST}/candidate/{id}

where {id} is the integer identifier of the challenge (not its name). No authentication is required. Example output:

  "id": 3,
  "proof_of_knowledge": "DA95FB593E561E3D24BA7C6AFFE28169EF17A76BCE9442E6EE1E768E5253E2844104C6513DD37DF82E7600018ACA5BC2AF467129DEF7903EDA693C0BBB76BD55",
  "public_key": "7D333C24CE6E539C66E6C7EF159CC1FA75F1B36321D4E6F4E3C0B42294D927A0C77DCFDE5782CF5B82B18C3499575D5191950F56678AE4F7B11815FA19A66807",
  "test_vectors": [
      "hash": "CA432252911CB7D6984354F2C08B94850C013C4A7584C53E044D5AA840234A82",
      "signature": "F946FA83BEBB4F1C31729175034A2971C1C8E9577DD7AB74DA476854157F459436A4C9EFE55DA6DC9E58B99777C6DD47F925C75B33A045F9F2E22DE7B26F6F24"
      "hash": "E8720F0770A55ECF716B8E7E5EDFC7DE29A0D60D34FB22D999B6652F9EFC5092",
      "signature": "EBDA369AABA4C9C1D234A089C9E8C4A5C58474B281FB03F87DDB796A8379604BAD85EFB1FFAAB2866F5F58BCA2B10F9837598C09EC46E739D78EBB5E0A0F05E7"
      "hash": "0011F7845B3B0177A8FFA0B091EF6E18CBAC48B9CB1D094AA132D115B29A3C17",
      "signature": "2DFCE93EF82D986F4D70D9A13DCAE09710251EBC410C5C9D40D2A58DB2072422AB5D507051B6E84C76D7EB0C9171663D17D0ADE206B06D5CA32C212BF7CA9202"
      "hash": "74EE4AC38DAFE9E412A6BFD4B5C700D323647DF08E5D9CA6424FB9545AF8A921",
      "signature": "4ACADB17EEA49231B8D418B4183F60108687A545C51329F8305C975CB0FA8BE745A668E46EA2667391044811C37C3B66E372DD7F85E026BCBCE5269E966153AD"
      "hash": "635E98797C020703DF02401FC7875F6A898DAF6A97B44545085CEB7121DBF87C",
      "signature": "936F5F1BB888D9767C85F35FD1E2D3B7E00B756CF1132CB3BC5D11E4F1FB9566DBB386D66EBBA03D08B8CCBCCFDCDEF9B347A3F0FA532FF863631517892631C7"
      "hash": "6A80250B5D99AB19E7A22CD0C693DB5512171BA52F4CD6E5ED57D0F235BE5744",
      "signature": "56C083E3EB85F6A7CEF35759717DC22CC3680A0347AC438462B58560D68EAA083E76468601FD86E4CB0AC3DCD72B76193193A91431ABB709F77106AB1E1004D6"
      "hash": "C27A231D7FA08308DDFD708E1164885BC69126E6DEC7362388AF6E9AB5A9011F",
      "signature": "15D5D181EE52D3946B720D4DBF6A1EF71D0A4DAB88B7D5339C1D74661029EB899624CF95A91163FCA4561091E780B7B677095EE3CF1F84FA268C6BEB299FADF4"
      "hash": "50365D99155091C54E137CC3E7FB0B706BB0EFED921F690354F4AA08D783C94C",
      "signature": "3F90C0F2912B52CEC781ADCF6E489B636549E543DBCE133D63B8B1B5E620291D5172A8A5BA844FB56E33C7D2324369DAF0BD29AC7FA0CDDD823EB9911577C71D"
      "hash": "4E9A84C83082848926BF0B89E7D3585BE767C8657B4FFBAA9370C9839EC4D26E",
      "signature": "5C6135E8BEDC689177CFE3AA25E6ED1EB8904DEB7E5967DDE901EAA3C5AE4A8D6A5C8BEB4A350E9A7828CCDC6904893CD16F2721D14320F9FE95C507F58B8B00"
      "hash": "743E576BD584EEA6EDFE32CD3ECC3BB6C013258539CC31C653C31F7371E72A90",
      "signature": "15A68010CF6EF4C2C29E9549D20532B4E86BECB18AA2C3338BCDE8081BF634F87FC5610E6978072CF8851ECBBF0377084E12B23130A8A2F84062C81B8227DFE7"

Performance scores

The performance score of a challenge is measured by the SYSTEM when validating its consistency at posting time. Challenges with a higher performance score collect STRAWBERRY points faster. The score is assigned as follows.

Execution time. The SYSTEM measures the average CPU time consumed by the challenge program and determines the fraction \[f_{\text{time}} = \frac{\text{average time}}{3 \text{ seconds} }\;.\]

Code size. The SYSTEM measures the size of the executable and determines the fraction \[f_{\text{size}} = \frac{\text{code size}}{20 \text{ MB} }\;.\]

RAM usage. The SYSTEM measures the average RAM consumption of the executable and determines the fraction \[f_{\text{memory}} = \frac{\text{average RAM}}{20 \text{ MB} }\;.\]

The performance score of the challenge is evaluated as \[\mathtt{perf\_score} = \log_2 \left(\frac{2}{f_{\text{time}} \cdot f_{\text{size}} \cdot f_{\text{memory}}}\right)\;.\]

Note that the performance score of a challenge that meets the allowed maximal limits is equal to \(1\). Challenges that are either smaller, faster or less memory-consuming get a higher perfomance score.

Winning strawberries

A contestant may post challenges anytime between the STARTING DATE and the POSTING DEADLINE.

A challenge gets more and more STRAWBERRIES as time goes by, as long as it is not BROKEN. In this third edition of the competition, time has a granularity of 1 minute instead of 1 day.

At the time \(t_{post}\) the challenge is posted, its STRAWBERRY score is set to \(0\). The STRAWBERRY score then increases quadratically with time as \[\mathtt{strawberries} = \mathtt{perf\_score} \times \left(\frac{t-t_{post}}{24\cdot 60}\right)^2\] where \(t-t_{post}\) is the number of minutes elapsed since the challenge was posted and \(\mathit{perf\_score}\in[1,+\infty)\) is its performance score (see above).

If the challenge is BROKEN for the first time at time \(t_{break}\), its STRAWBERRY score becomes \[\mathtt{strawberries} = \mathtt{perf\_score} \times \left(\frac{t_{break}-t_{post}-\left(t - t_{break}\right)}{24\cdot 60}\right)^2 \;,\] meaning that its progression is reversed symmetrically until it reaches \(0\). It then sticks to it.

When the FINAL DEADLINE is reached, the STRAWBERRY scores of all challenges freeze.

Breaking challenges

A contestant may BREAK any challenge by submitting a candidate key to the SYSTEM. The challenge may be STANDING, or already BROKEN.

Key verification procedure: Given the submitted key, the SYSTEM

  1. generates the public key corresponding to the submitted private key,
  2. fetches the public key of the target challenge,
  3. 3. verifies that the two public keys match.
The submitted private key is erased as soon as the verification is completed.

In case of mismatch, the break is rejected. Otherwise, the submitting contestant is notified that the break is accepted. If the challenge was STANDING, it is declared as BROKEN at the time the SYSTEM accepted the break.

Contestants may BREAK challenges until the FINAL DEADLINE.

Winning bananas

Every newly registered user, referred to as a contestant, is assigned a BANANA score initialized to 0.

A contestant whose BREAK has been accepted by the SYSTEM gets a chance to increase their BANANA score. Noting \(\mathtt{strawberries}\) the current STRAWBERRY score of the challenge and \(\mathtt{bananas}\) the contestant's current BANANA score, \(\mathtt{bananas}\) is updated as \[\mathtt{bananas} = \max(\mathtt{bananas}, \mathtt{strawberries})\;.\]

Winning the competition

The winners are determined at the time of the FINAL DEADLINE. There are 2 winners, the STRAWBERRY WINNER and the BANANA WINNER.

The strawberry winner

The WINNING CHALLENGE is the challenge (STANDING or BROKEN) which, by STRAWBERRY score, has reached the highest peak between the STARTING DATE and the FINAL DEADLINE.

The STRAWBERRY WINNER is the contestant who posted the WINNING CHALLENGE.

The banana winner

The BANANA WINNER is the contestant with the highest BANANA score at the time of the FINAL DEADLINE.

Cash prizes

A 2000$ cash prize will be shared between the winners, as follows:

  • Designers' podium:
    • 500$ for the strawberry winner
    • 200$ for the second highest strawberry score
    • 100$ for the third highest strawberry score
  • Attackers' podium:
  • 500$ for the banana winner
  • 200$ for the second highest banana score
  • 100$ for the third highest banana score
  • Bonus for the designers' **or** attackers' podium
  • 250$ for the winner
  • 100$ for the second
  • 50$ for the third

The bonus cash prizes go to the designers' podium if there is at least one STANDING challenge at the FINAL DEADLINE. The bonus cash prizes go to the attackers' podium if all the challenges are BROKEN at the FINAL DEADLINE.


At any time, the ORGANIZING COMMITTEE may DISQUALIFY a contestant in case of misconduct during the competition. Examples of misconduct include

  • posting a challenge program that does not produce a verifiable ECDSA signature on P-256 curve with probablity 1,
  • posting a challenge program that contains malware,
  • attempting to attack/hack the SYSTEM or the computer system of contestants in any manner.

The user account of a DISQUALIFIED contestant is disabled and challenges that the contestant has posted may be withdrawn from the competition on a case-by-case basis.

Terms of reference

Important dates
STARTING DATEMay 17, 2021 @ 00:00 UTC
POSTING DEADLINEAugust 22, 2021 @ 00:00 UTC
FINAL DEADLINESeptember 11, 2021 @ 12:00 UTC (the days before CHES 2021)
System and challenges
SYSTEMServer comprising the competition website and tools for compiling and testing challenge programs. The compilation environment is fully described in this dockerfile.
REFERENCE COMPILER gcc version 8.2.0
REFERENCE IMPLEMENTATIONdECDSA.c file available from this GitHub repository.
BROKEN (challenge) At least one contestant has been able to provide the SYSTEM with the private ECDSA key that passes the key verification procedure.
STANDING (challenge) A challenge that is not BROKEN.
Winning contestants
WINNING CHALLENGE Challenge which STRAWBERRY score has reached the highest peak between the STARTING DATE and the FINAL DEADLINE.
BANANA WINNER Contestant with the highest BANANA score at the time of the FINAL DEADLINE.
DISQUALIFIED (contestant) Misconducting contestant excluded from the competition and whose challenges may be withdrawn.
Organizing committee
Twitter@WhiboxC (WhibOx Contest Organizing Committee)